The acitons of pertussigen (pertussis toxin (Ptx)) on the IgE- containing spleen cells of C57B1/10SN mice, and on the promotion of protective activity of vaccines made from Bordetella species were studied. Mice receivint Ptx and 1 mg of egg albumin (EA) showed a marked increase in the percentage of spleen cells containing surfac IgE. This action was more pronounced in older mice (10 weeks or older) than in younger mice (5-6 weeks old). Since the increase of IgE-containing cells was as high as 40% of the total spleen cells, it seems that the main action of the Ptx was that of increasing the receptors for IgE in the B-cell population rather than just increasing the IgE-producing cells. When Ptx was given alone, only a slight increase of IgE-containing cells was observed, and EA alone did not have any effect on the number of these cells. Ptx increased strikingly the protective action of vaccines made from B. pertussis strains lacking Ptx, or from strains in which a genetically modified gene for the S1 subunit of Ptx was present. Ptx also enhanced the protective activity of vaccines made from B. parapertussis and B. bronchiseptica, but had little effect on vaccines made from wild-type strains of B. pertussis known to produce Ptx. The antibody response to surface antigens of Bordetella has been studied. We have shown that the antibodies of the IgG class is not increased. However, the IgE class of antibodies was noticeably increased in vaccines containing Ptx or when Ptx was included in the immunization with vaccines lacking Ptx.